Polarized s-quark Distribution in Charmed Hadron Leptoproduction

6 Pages·0.08 MB·English

by  Kazutaka SUDOH

KOBE–FHD–02–02
January 2002
Polarized s–quark Distribution
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6 Kazutaka SUDOH
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Graduate School of Science and Technology, Kobe University,
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5 Nada, Kobe 657–8501, JAPAN
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E-mail: [email protected]
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ABSTRACT
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In order to extract the polarized strange quark density in proton, we studied
ar the semi–inclusive Λ+/Λ¯+ leptoproduction in charged current DIS at THERA en-
c c
ergies. We indicate that measurements of the spin correlation between the incident
proton and the produced Λ+/Λ¯+ baryon gives us information about the polarized
c c
strange quark distribution.
PACS numbers: 13.60.-r, 13.88.+e, 14.20.Lq
†Talk presentedat SPIN2001(The 3rdCircum–Pan–PacificSymposium on“High EnergySpin
Physics”), Beijing, China, Octobar 2001.
1 Introduction
Protonspin puzzle iscurrently oneof themost challenging topics inhighenergy spin
physics. As is well known, proton spin is composed of the spin and orbital angular
momentum of quarks and gluons. The polarized parton distribution function (PDF)
plays an important role on deep understandings of the spin structure of proton.
However, knowledge about the polarized sea quark distribution remains still poor.
In order to understand the spin structure of proton, we need more information
about the polarized sea quark distribution functions. Recently, HERMES group has
reported [1] that direct measurement of the strange sea is required to explain the
violation of the Ellis–Jaffe sum rule [2]. So far, there are several parametrization
models of the strange quark distribution. Though the most simplest case is to
assume the flavor SU(3) , a new parametrization taken account of the violation of
f
SU(3) is also recently proposed [3].
f
The study of heavy quark production in deep inelastic scattering (DIS) is one
of the promising way to access the parton density in proton, since heavy quark is not
main constituents of proton and is only produced in hard processes. Heavy quark
leptoproduction inneutral current DIShasalready measured by somecollaborations
at HERA. However, no measurement of heavy quark production in charged current
DIS has been so far done at HERA due to the small cross section. We can expect
that the cross section for charged current heavy quark production will be several
times larger at THERA than at HERA [4]. THERA is a future high energy ep
collider which uses electron or positron from the linear collider TESLA at energies
of 250 GeV to 800 GeV and proton from HERA at energy 920 GeV. Thus, charged
current heavy quark production is also interesting and challenging topics.
In this work, to examine the polarized s/s¯quark distribution, we have studied
semi–inclusive Λ+/Λ¯+ leptoproduction in charged current DIS; l++p~ ν¯+Λ~++X,
c c → c
l− + p~ ν + Λ~¯+ + X, where arrows attached to particles mean that these parti-
→ c
cles are polarized. These processes might be observed in the forthcoming THERA
experiments. Since in the naive quark model, the Λ+ baryon is composed of a
c
heavy c–quark and anti–symmetrically combined light u and d–quarks, we can as-
sume the polarization of Λ+ baryon to be the one of charm quark. In addition, Λ+
c c
is dominantly produced by the fragmentation of charm quark which is originated
±
from strange quark through the t–channel W boson exchange at the leading order.
Therefore, there can be a correlation between the strange quark polarization and
the produced charm quark polarization. Hence we can expect that the measurement
of the spin correlation between the incident proton and the produced Λ+/Λ¯+ gives
c c
us information about the polarized s/s¯ quark distribution in proton.
2
2 Double Spin Asymmetry
For above processes, we calculated the double spin asymmetry AH for final state
LL
hadron specified by H (Λ+ or Λ¯+), which is given by
c c
AH [dσ++ −dσ+− +dσ−− −dσ−+]/dpT = d∆σ/dpT, (1)
LL ≡ [dσ++ +dσ+− +dσ−− +dσ−+]/dpT dσ/dpT
where dσ+−, for instance, denotes that the helicity of proton is positive and the one
of H is negative. p is a transverse momentum of final hadron H.
T
For Λ+ production, the spin–dependent differential cross section at leading
c
order level can be generally written by
d∆σˆ
d∆σ(l+p ν¯Λ+X) = U2 ∆s(x)+U2 ∆d(x) dx dtˆ∆DΛ+c (z)dz, (2)
→ c cs cd dtˆ ! c
n o
where ∆s(x) and ∆d(x) are the polarized s–quark and d–quark distribution func-
tions, respectively. U and U are CKM parameters. ∆DΛ+c (z) is the polarized
cs cd c
fragmentation function of outgoing charm quark decaying into Λ+. We used the
c
model of Peterson et al. [5] as the unpolarized fragmentation function [6]. Unfor-
tunately, we have at present no data about the polarized fragmentation function
because of lack of experimental data. By analogy with the study on Λ polarization
[7], we took the following ansatz:
∆DΛ+c (z) = CΛ+c (z)DΛ+c (z), (3)
c c c
where CΛ+c (z) is the scale–independent spin transfer coefficient. Here we apply the
c
analysis on Λ production to Λ+ production and choose the following two models;
c
CΛ+c (z) = 1 (the naive nonrelativistic quark model) and CΛ+c (z) = z (the jet frag-
c c
mentation models [8]) to evaluate cross sections.
The double spin asymmetry AΛ+c is described in terms of the spin transfer
LL
coefficient CΛ+c (z) and the ratio of the PDF’s as
c
AΛ+c CΛ+c (z){U2cs∆s(x)+U2cd∆d(x)}. (4)
LL ∝ c U2 s(x)+U2 d(x)
{ cs cd }
Thus AΛ+c is proportional to linear combination of the polarized s–quark and d–
LL
quark distribution function. Since the contribution from the valence d–quark is
large, the s–quark distribution is not so clearly extracted in Λ+ production case.
c
On the contrary, AΛ¯+c in Λ¯+ production can be represented as
LL c
AΛ¯+c CΛ+c (z) U2cs∆s¯(x)+U2cd∆d¯(x) = CΛ+c (z)∆s¯(x) CΛ+c (z)∆s(x), (5)
LL ∝ c n U2 s¯(x)+U2 d¯(x) o c s¯(x) ≃ c s(x)
cs cd
n o
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¯
in which the d–quark contribution and CKM parameters can be eliminated, and
then AΛ¯+c is directly proportional to the strange quark distribution function alone.
LL
Therefore, we can clearly extract the polarized strange quark distribution ∆s(x).
Note that above equation is derived in both cases; the flavor SU(3) case (∆u¯(x) =
f
¯ ¯ ∗
∆d(x) = ∆s¯(x)) and the non–SU(3) case (∆u¯(x) = ∆d(x) = λ∆s¯(x) ).
f
3 Numerical Results
Settingacharmquarkmassm = 1.5GeVandtherelevantcollider energy√s = 300
c
GeV, we numerically calculated the spin–independent and dependent differential
cross sections and the double spin asymmetry. As for the PDF’s, we used the
GRV98[9]parametrizationastheunpolarizedPDF,andtheAAC[10]and“standard
scenario” of GRSV01 [11] parametrizations for the polarized one.
We show the double spin asymmetry in Fig. 1 as a function of p at √s = 300
T
GeV for Λ+ production (left panel) and for Λ¯+ production (right panel). In order to
c c
suppress the contributions from the diffractive process and higher twist corrections,
we have imposed the kinematical cut on p as p > 2 GeV in numerical calcula-
T T
tions. In figures, the bold and normal lines show the case of AAC parametrization
and “standard scenario” of GRSV01 parametrization, respectively. The solid lines
represent the spin transfer coefficient CΛ+c (z) = 1 case, while the dashed lines rep-
c
resent CΛ+c (z) = z case.
c
Asshowninfigures, AΛ+c insmaller p regionsdoesnotdependonthemodelof
LL T
polarized PDF’s, and is strongly affected by the shape of the spin transfer coefficient
CΛ+c (z). Therefore, The Λ+ production is effective for extracting information about
c c
thepolarizedfragmentationfunction∆DΛ+c (z). Inaddition, wecandirectlymeasure
c
thepolarizedstrangequark distributionfunction, thoughtheparametrizationmodel
dependence is not so large. On the other hand, for Λ¯+ production AΛ¯+c for GRSV01
c LL
parametrization rapidly decreases in larger p regions, and we see big difference
T
between two parametrization models. Therefore, measuring AΛ¯+c in largerp regions
LL T
is quite effective for testing the model of polarized PDF’s, since the ambiguity of
the polarized fragmentation function becomes small.
4 Summary
Insummary, toextractinformationaboutthepolarizedstrangequarkdistributionin
proton, the semi–inclusive Λ+/Λ¯+ production in charged current DIS in unpolarized
c c
∗λ representa degree of SU(3) violationand is a parameter which should be determined from
f
experiments [3].
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lepton–polarized proton collisions was studied. Λ¯+ production is most promising
c
not only for testing the parametrization models of PDF’s but also for directly ex-
tracting the polarized strange quark distribution ∆s(x) by measuring the double
spin asymmetry AΛ¯+c .
LL
0.4 0.4
√s = 300 GeV GGRRSSVV0011 CCcΛΛ((zz))==z1 √s = 300 GeV GGRRSSVV0011 CCcΛΛ((zz))==z1
AAC CΛ(z)c=1 AAC CΛ(z)c=1
0.3 AAC CcΛ(z)=z 0.3 AAC CcΛ(z)=z
c c
Λc Λc
LL 0.2 LL 0.2
A A
0.1 0.1
0 0
20 40 60 20 40 60
p [GeV] p [GeV]
T T
Figure 1: p distribution of double spin asymmetries at √s = 300 GeV for Λ+
T c
production (left panel) and for Λ¯+ production (right panel).
c
References
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